The present invention relates to the prevention of fouling of electrodes used to measure or monitor the pH of the bacteria-containing liquid of an anaerobic digestion process.
Many processes are in use today wherein the decomposition of organic molecules is accomplished through the use of anaerobic bacteria. Such processes are known by various names such as bioconversion processes, anaerobic decomposition processes, anaerobic digestion processes, anaerobic filters, anerobic reactors, and the like. Herein, the term "anaerobic digestion process" will be applied to these processes wherein anaerobic bacteria decompose organic molecules to mainly carbon dioxide and methane. These anaerobic digestion processes are utilized for various purposes, in some cases the primary function or desired result being the decomposition of an organic molecule which itself presents a disposal problem; while in some cases the primary function is the production of methane as an energy source.
In an anaerobic digestion process, generally two different bacteria populations live in balance, one group being known as the acid formers, and one group known as the methane formers. The acid formers decompose relatively large molecules to small C.sub.1 -C.sub.4 organic compounds such as formic acid, acetic acid, propionic acid and butyric acid. The methane formers then further decompose the C.sub.1 -C.sub.4 molecules into carbon dioxide and methane. The anaerobic bacteria known as the methane formers are especially delicate and sensitive to pH, and the pH of a liquid in which an anaerobic digestion process takes place must remain within a narrow range of about 5 to 8, more usually within the range of about 6.2 to 7.5, in order to prevent killing of the bacteria.
Due to the sensitivity of an anaerobic digestion process to pH, the pH of the anaerobic bacteria-containing liquid of the process is generally measured or monitored on a continuous or frequent intermittent basis in order that any adverse change in pH may be detected and corrected. This is accomplished through use of a pH meter or other pH measuring device having electrodes which are immersed in the bacteria-containing liquid. Usually a small sample side-stream of the liquid from the main anaerobic reactor is passed through a cell or chamber housing the electrodes such that the electrodes (the sensing areas thereof) are immersed in the liquid flowing through the chamber. The sample side-stream is then generally discarded in a large commercial process, but may be returned to the reactor in a small laboratory or pilot plant operation.
A problem arises in the pH measurement of the bacteria-containing liquid because the bacteria in the liquid very quickly cause a gel-type film to form and grow on the surface of the sensing membrane of the pH glass electrode, and on the porous plug of the reference electrode. This film causes an increased electrical resistance which results in an inaccurate pH measurement. The inaccuracy in the pH measurement is frequently termed as "drift". In a typical anaerobic digestion process the film buildup will require electrode replacement after only two or three days of operation.
Various techniques have been devised for cleaning of pH electrodes used in other processes. These cleaning systems, which are commercially available, include mechanical systems wherein a brush or scraper physically removes fouling; chemical spray systems wherein an acid, base or emulsifier is periodically sprayed on to the electrode; hydrodynamic systems wherein turbulence is induced in the sample flow to effect cleaning through abrasive action of suspended solids contained in the sample flow; and acoustical systems wherein cleaning is accomplished through use of ultrasonic waves. None of the prior art systems provide effective cleaning of the gel-type film caused by anaerobic bacteria. For example, the brushes and scrapers of a mechanical system will not clean the pores of the porous plug of the reference electrode; and the chemical spray system appears to be mainly effective for prevention of inorganic deposits instead of those induced by the anaerobic bacteria. Ultransonics and hydrodynamics have also been found to be unsatisfactory.
It is thus an object of the present invention to provide a new and effective method or system for prevention of fouling of pH electrodes used in an anaerobic digestion process. It is an additional object of the present invention to provide a method or system for periodic removal of the film on pH electrodes caused by anaerobic bacteria of an anaerobic digestion process. It is a further and additional object of the present invention to provide a method for intermittent treating of pH electrodes used in an anaerobic digestion process, and which method allows calibration of the pH measuring device during the treating. Additional objects will become apparent from the following description of the present invention.